When I was young, my Mom used to call my brother and one of his friends Fric and Frac. They looked nothing alike, but where you found one, you found the other. For the most part, they were interchangeable- If one of them wasn't figuring out what trouble to get into, the other was. Not real trouble- just the usual shenanigans boys got into 40 years ago.
I find myself sometimes thinking of Hope and GB as Fric and Frac. They like many of the same things, and the best (worse?) consequence I can give them is to insist they play in separate rooms. They complain about each other using the same words, which is partially caused by Hope's tendency to mimic GB constantly, but also comes from their common lack of social skills. The constant "She's looking at me" gets old, but not as aggravating as the realization that I let them out of my sight and there is nothing but silence. They know they are not allowed out of sight and that it won't last long, so they take advantage of it whenever I let it happen. Once it was talcum powder all over MK's bathroom, another time it was Fric, Frac and Ellie eating baking chips behind the couch. They have painted the radiators with nail polish and "baked" with a combination of moon sand and brownie mix.
It is never more than a couple of minutes and it doesn't happen as often as it did in the beginning (it takes forever to clean up talcum powder, especially with their help) but during those times, I think the best bonding occurs between them... Fric and Frac vs. the Mom.